Measurement of low optical losses in an absorbing medium is commonly performed by applying a light signal to a long absorbance path length, using single-pass or multi-pass paths in the absorbing medium. The signal is measured as a function of the desired parameter, such as wavelength of the light, time, concentration, etc., by a light detector, in comparison with a reference channel. Such a light detector normally terminates the light path of the signal and the reference channel. These measurements require either light sources with very low variations in intensity or a good reference scheme. In particular, the sensitivity of the single or multipass absorption techniques is limited by the pathlengths that can be achieved, the sensitivity of the detector towards small changes in transmission, and the temporal and spatial stability of the signal, which in turn is influenced by the temporal and spatial stability of the light source and of the reference vs. signal channel, the alignment of the source, the medium and detector, and of the detector and associated equipment. As a result, measurement of low optical losses using such technique is difficult and yields unreliable data.
The use of ring-down time of a light signal in a cavity consisting of mirrors can also be used to measure optical characteristics of an absorbing medium. Such optical cavities consist of two or more mirrors, between which an optical signal is reflected to characterize the mirrors as well as the optical characteristics of an absorbing medium (e.g., gases, molecular beams, etc.) between the mirrors (Romanini et al. 1993; Scherer et al. 1997; Berden et al. 2000; Lehmann, U.S. Pat. No. 5,528,040, issued Jun. 18, 1996).
A ring-down cavity has been set up with a crystal inside a cavity defined by mirrors, and the spectra of compounds at the surface of the crystal have been measured using evanescent wave spectroscopy (Pipino et al. 1997). Also, the crystal faces have been used to define a cavity and thereby create a cavity without mirrors, wherein the signal rings down due to internal reflection of the crystal. This technique has also been used for evanescent wave spectroscopy (Pipino, U.S. Pat. No. 5,835,231, issued Nov. 10, 1998).
Although cavity ring-down spectroscopy (CRDS) is well established as a gas phase method, applications in condensed phase have, until recently, been limited to absorption measurements of films through evanescent field experiments on the surface of all-solid state cavities (Pipino et al. 1997) and to films deposited on windows inside the cavity (Engeln et al. 1999).
Very recently CRDS was shown to be applicable to absorption measurements on liquid samples, in which a high finesse cavity was either filled entirely with a liquid sample (Hallock et al. 2002) or the liquid was contained in cuvettes (Xu et al. 2002). To our knowledge, there have only been two previous attempts at ring-down measurements using optical fibers. Von Lerber et al. (2002) constructed a cavity by depositing highly reflective coatings onto both fiber end facets of a 10 m optical fiber. Stewart et al. (2001) inserted a gas phase absorption cell into a fiber-loop, leading to very high transmission losses. These losses necessitated the use of a fiber amplifier, and the sensitivity of measurements using such an active loop depended strongly on the amplifier's temporal stability.